Silicon-on-Insulator (SOI) structures are frequently used in Complementary Metal-Oxide-Semiconductor (CMOS) applications.
Such structures comprise, from their useful surface to their bottom side: a thin silicon layer; a buried layer made of a dielectric material that is typically an oxide, such as SiO2; and a support substrate. The buried layer made of a dielectric material is often denoted by the acronym BOX for the term “Buried OXide.”
The thicknesses of the thin silicon layer and the oxide layer may vary depending on the intended applications.
In particular, the thickness of the thin silicon layer is reduced to a thickness of 50 nanometers (nm) or less, even 20 nm or less, and especially to about 12 nm in order to allow what are called “FDSOI” (“Fully Depleted SOI”) structures to be obtained. Such FDSOI structures have the advantage of significantly reducing operational instability and considerably improved performance relative to what are called “PDSOI” (“Partially Depleted SOI”) structures in which the thickness of the thin silicon layer is about 70 to 90 nm. The improved performance may include low dynamic power, low leakage current, and/or high transistor density.
Among these structures, UTBOX (UTBOX standing for “Ultra-Thin Buried OXide”) structures, having an ultrathin buried oxide layer, show great promise because the extreme thinness of this electrically insulating layer makes it possible to apply a voltage to the back side of the structure (i.e., to the side opposite the thin silicon layer) and therefore to precisely control the operation of the device.
The term “ultrathin” is understood to mean having a thickness of 50 nm or less.
The fabrication processes of structures having a buried oxide layer with a thickness of between 25 and 50 nm are, at the present time, quite well characterized and it is possible to produce such structures with a defectivity level that is compatible with subsequent component fabrication.
However, UTBOX structures with a buried oxide layer having a thickness of 25 nm or less, and particularly of 15 nm or less, can currently only be fabricated with a defectivity level that is not easily compatible with the requirements of component manufacturers.
More precisely, this defectivity is due to a bubbling or blistering effect that is observed at the bonding interface located between the thin silicon layer and the mechanical support, in the case of an SOI substrate fabricated using the SMART CUT® process.
FIG. 1 shows the variation in the defectivity as a function of the thickness of the BOX layer expressed in nanometers (nm).
The defectivity shown in this graph is the number of bubbles counted on the surface of an SOI structure immediately after the thin silicon layer has been transferred.
In the case of structures in which the BOX layer has a thickness below 15 nm (the hatched zone of the line in FIG. 1), the bubbling is so widespread that it may be impossible or impractical to count the bubbles.
FIGS. 2A to 2D illustrate the main steps of a first known process for fabricating such a structure employing the SMART CUT® process.
With reference to FIG. 2A, an oxide layer 2 is formed on the surface of a donor substrate 31 from which the thin silicon layer will be transferred.
A weak zone 32 is formed in the donor substrate 31, at a depth corresponding to the thickness of the thin layer 3 to be transferred, through the oxide layer 2, for example, by implantation of atomic species (represented by the arrows in FIG. 2B).
With reference to FIG. 2C, the donor substrate 31 (by way of the oxide layer 2) and the receiver substrate 1 are hydrophilically bonded by molecular adhesion.
This bonding step is followed by a bond-strengthening anneal intended to increase the bond strength.
Next, a supply of energy, for example, thermal energy, causes the donor substrate 31 to cleave in the weak zone 32.
In general, the bond-strengthening anneal is carried out at a low temperature (i.e., at a temperature between 200° C. and 550° C.) and the anneal allows the bonding interface to be strengthened and the cleaving of the donor substrate 31 to be initiated in the same step.
After the non-transferred part of the donor substrate 31 has been detached, a silicon-on-insulator structure (FIG. 2D) is obtained to which conventional finishing treatments (rapid thermal annealing (RTA), sacrificial oxidation, etc.) are applied, these treatments being intended to, inter alia, smooth the surface of the thin semiconductor layer 3 and repair implantation-related defects.
The one or more RTA treatments are typically carried out at a temperature above 900° C.
In the step of bonding the two substrates, water molecules present at the interface contribute to the bonding of the surfaces.
However, during the bond-strengthening anneal, water molecules diffuse through the oxide layer 2, and through a thin native-oxide layer on the surface of the receiver substrate 1, and react with the silicon of the semiconductor layer 3 and with the silicon of the receiver substrate 1. The reaction between the water molecules and the silicon may proceed as shown in the following oxidation reaction:2H2O+Si→SiO2+2H2 
This reaction produces molecules of hydrogen gas, which are trapped in the buried oxide layer, the buried oxide layer thus acting as a hydrogen-gas reservoir.
However, in the case of an ultrathin oxide layer, the layer is not thick enough to store all of the hydrogen gas molecules.
The buried oxide layer therefore becomes saturated and can no longer absorb the molecules of hydrogen gas. The excess hydrogen accumulates at the bonding interface where it generates defects.
This is because, as soon as the temperature of the bonded structure exceeds about 300° C., the hydrogen gas subjects defects present at the bonding interface to pressure, forming bubbles.
This effect is described in the following articles: “A model of interface defect formation in silicon wafer bonding,” S. Vincent et al., Applied Physics Letters, 94, 101914, (2009); and “Study of the formation, evolution, and dissolution of interfacial defects in silicon wafer bonding,” S. Vincent et al., Journal of Applied Physics, 107, 093513, (2010).
By carrying out an anneal at a temperature of between 300° C. and 400° C., the generation of hydrogen gas is limited and thus the bubbling effect is prevented.
Thus, after the cleaving, a structure with a very low defectivity is obtained.
However, the bonding interface still needs to be adequately strengthened and the SOI substrate still needs to be finished without allowing bubbles to appear in the finishing steps.
At temperatures of 900° C. and above, hydrogen gas is soluble in silicon.
After the cleaving, the objective is therefore to increase the temperature to 900° C. (a temperature above which hydrogen outgases from the silicon) rapidly enough to set the structure and thus to prevent generation of defects at the bonding interface.
However, after a conventional RTA treatment micro-bubbles are observed to form in the structure and, although these bubbles are much smaller than those observed after the known process described with reference to FIGS. 2A to 2D, they make it impossible to use the structures for the intended applications.
This is a result of the fact that during the RTA the temperature was not increased rapidly enough to set the structure and prevent the bubbling effect.
It is therefore still necessary to develop a process that prevents bubbles from forming in the case of structures in which the BOX layers are 15 nm or less in thickness, and, in particular, 10 nm or less in thickness.
To prevent H2 from forming, document WO 2010/049496 describes a second process, the steps of which are represented in FIGS. 3A to 3E.
With reference to FIG. 3A, an oxide layer 21 is formed on the surface of the donor substrate 31.
A weak zone 32 is formed within the donor substrate 31 at a depth corresponding to the thickness of the thin layer 3 to be transferred, through the oxide layer 21, for example, by implantation of atomic species (represented by the arrows in FIG. 3B).
With reference to FIG. 3C, an oxide layer 22 is formed on the surface of the receiver substrate 1.
Next, molecular adhesion (oxide/oxide) bonding is used to bond the donor substrate 31 to the receiver substrate 1, the oxide layers 21, 22 being located at the interface and together forming the buried oxide layer 2 of the SOI substrate.
After this bonding step, the donor substrate 31 is cleaved at the weak zone 32.
This process achieves good results in terms of defectivity in so far as the H2-generating reaction is limited by the presence of the two facing buried oxide layers 21, 22 that form a barrier to water-molecule diffusion.
Specifically, these molecules cannot reach the oxide/silicon interface, the silicon oxidation reaction cannot take place, and the generation of H2 molecules is thereby prevented.
However, bonding substrates by way of their respective oxide layers 21, 22 have the drawback that the bonding interface is not completely closed. In other words, when the structure is observed with a transmission electron microscope after the finishing anneals (RTA at 1200° C. for 30 seconds), the interface between the two layers (represented by the dot-dash line 23 in FIG. 3E) may still be seen.
This incompletely closed interface may create electrical problems that could interfere with the operation of the electronic devices formed in or on the structure.